Basal Cell Adenoma of Palate, a Rare Occurrence with Review of Literature.

Basal cell adenoma is an uncommon benign epithelial neoplasm of salivary gland which derives its name from the basaloid appearance of tumor cells and accounting for 1-2 % of all salivary gland epithelial tumors. This tumor usually arises in the major salivary glands, with the parotid being the most frequent site of occurrence, followed by the upper lip; while it is very rare in the minor salivary glands. Microscopically, it is composed of isomorphic cells similar to basal cells with nuclear palisading. We report a case of BCA presenting as an asymptomatic swelling over the right side of palate of 55-year-old female patient. A follow-up of 1 year revealed no recurrence. This report emphasizes the rare site of occurrence of this tumor and briefly reviews the literature.


Introduction
World Health Organization defined basal cell adenoma (BCA) as an idiosyncratic benign neoplasm composed of monomorphic population of basaloid epithelial cells, organized with a prominent basal cell layer and a distinct basement membrane like material; however, the myxochondroid stromal component characteristic of mixed tumor is absent. [1][2] Kleinsasser and Klein (1967) first used the term basal cell adenoma to describe an encapsulated, slow growing, merely epithelial neoplasm composed of discernible basal cells arranged in the form of solid sheets or nests and trabecular/ tubular cord like pattern. [3] The most common site of occurrence is the parotid gland [4][5] followed by upper lip with a decreasing incidence in palate, buccal mucosa and lower lip. [4] It usually occurs in patients over 50 years of age with slight female predilection. [6] Concerning its Clinical presentation, it exhibits as a slow growing, asymptomatic, movable, round or oval, normal-colored sub mucosal mass measuring less than 3 cm in diameter. [4] Microscopically, BCAs are well circumscribed and encapsulated by fibrous connective tissue. [1] Tumormass consists of proliferation of terminal duct epithelial cells forming is lands or sheets supported by a sparse fibrous stroma, and presence of a small number of myoepithelial cells. [4,2] It has distinct basement membrane and often exhibit palisading of basal layer cells.
They also lack the myxochondroid areas characteristic of pleomorphic adenoma. [7] Histopathologically, BCA can be divided into four subtypes, i.e. solid, trabecular, tubular, and membranous. [2] Basal cell adenoma is an uncommon tumor with palate being the rarest site for its occurrence. [4] To the best of our knowledge, only five cases involving the palatal minor salivary glands have been reportedin the English literature till date. The present case is most likely the sixth case [4,[14][15][16][17] which highlights the rarity of this tumor with regard to its site of origin/ occurence.

Case Report
A 55-year-old female reported to the department of Oral and Maxillofacial Surgery with a chief complaint of painless swelling on the right side of palate since two year, there were no signs of recurrence.

Discussion
The basal cell adenoma was once considered to be a type of monomorphic adenoma.  ute 2-6.5% of all the head and neck neoplasms [4,11] and tumors of minor salivary gland account for less than 25% of all salivary neoplasm. [4] Basal cell adenoma accounts for 1-2% of all salivary gland epithelial tumors. [1][2]7] BCA arises almost exclusively in adults and the mean age of the patient is 57.7 years with the peak incidence in sixth decade of life [1][2]; however, unusual cases of congenital BCAs have also been reported. [12][13] Several studies support a slight predominance of this tumor in females [8,11,[14][15] although membranous variant has an equal gender distribution. [1] Palate is the most common site for minor salivary gland tumors with 40-80% of incidence. [15] But it is relatively an uncommon site for BCA [14] which usually arises in the major salivary glands with the parotid being the most frequent site of occurrence followed by minor salivary glands of upper lip. [9, 16] In the opinion of Fantasia and Neville, BCA typically occurs in older patients with mean age of61 years and most commonly involves the upper lip. [17] In 1991, 160 cases of BCA were registered at the Armed Forces Institute of Pathology (AFIP), which constitute 1.8% of all benign epithelial salivary gland tumors, out of those BCA 75% were reported in the parotid gland and 20% in the minor salivary glands of the upper lip. [2,9] In our case the tumor presentation was in minor salivary glands of the palate which is considered as a rare site for BCA as only five cases have been reported in literature till date. [4,[14][15][16]18] In our case, the patient was female in sixth decade of her life which fits into the age range and gender pre- Ki67 staining of greater than 5% of the cells supports the diagnosis of BCAC. [10,19] The recurrence rate is 25-37% for the membranous variant of BCA, [9] possibly related to its multifocal nature, which impairs complete removal. [4] Although exceedingly rare, malignant transformation of BCA has been reported. [1] Therefore, it is necessary to perform complete tumor excision. This approach was used in our case and the postoperative period was uneventful with no signs of recurrence even after a follow up of 1 year.

Conclusion
Herein, the goal of this paper is to add on to the literature one more case of this rare tumor of BCA arising from minor salivary glands of hard palate and to sensitize the dental surgeons to include BCA as a differential diagnosis of palatal swellings.